A linear or two-dimensional ion-processing device such as an ion trap is formed by a set of elongated electrodes coaxially arranged about a central (z) axis of the device and elongated in the direction of the central axis. Typically, each electrode is positioned in the (x-y) plane orthogonal to the central axis at a radial distance from the central axis. The inside surfaces of the electrodes are typically hyperbolic with apices facing inwardly toward the central axis. The resulting arrangement of electrodes defines an axially elongated interior space of the device between opposing inside surfaces. In operation, ions may be introduced, trapped, stored, isolated, and subjected to various reactions in the interior space, and may be ejected from the interior space for detection. Such manipulations require precise control over the motions of ions present in the interior space, as well as over the geometry, fabrication and assembly of the physical components of the electrode structure. The radial excursions of ions along the x-y plane may be controlled by applying a two-dimensional RF trapping field between opposing pairs of electrodes. The axial excursions of ions, or the motion of ions along the central axis, may be controlled by applying an axial DC trapping field between the axial ends of the electrodes. Additionally, auxiliary or supplemental RF fields may be applied between an opposing pair of electrodes to increase the amplitudes of oscillation of ions of selected mass-to-charge ratios along the axis of the electrode pair and thereby increase the kinetic energies of the ions for various purposes, including ion ejection and collision-induced dissociation (CID).
Ions present in the interior space of the electrode set are responsive to, and their motions influenced by, electric fields active within the interior space. These fields include fields applied intentionally by electrical means as in the case of the above-noted DC and RF fields, and fields inherently (mechanically) generated due to the physical/geometric features of the electrode set. The inherently generated fields may or may not be intentional and, depending on the mode of operation, may or may not be desirable or optimal. The applied fields are not only governed by their applied operating parameters (amplitude, frequency, phase, and the like) but also by the size of the electrode set including the spacing between the electrodes. The inherently generated fields are also governed by the size and spacing of the electrodes. Both applied fields and inherently generated fields are governed by the configuration (profile, geometry, features, and the like) of the inside surfaces of the electrodes exposed to the interior space. Points on the inside surfaces closest to the central axis, such as the apical line of a hyperbolic electrode, have the greatest influence on an RF trapping field and thus on the ions that are constrained by the RF trapping field to the volume around the central axis.
In an ideal case, the physical features and geometry of the electrodes would be perfect such that no imperfections in the active fields existed and the fields would be uniform along the central axis of the electrode set. The electrodes would be perfect hyperbolic surfaces extending to infinity toward the asymptotes. The response of ions to the fields would be completely predictable and controllable, and the performance of the device as a mass analyzer or the like could be completely optimized. In practice, however, the electrodes contain a number of different features that engender various types of field faults or distortions that can adversely affect the manipulation and behavior of ions. For example, most electrode sets employed as ion traps eject ions from the interior space in a radial (x or y) direction orthogonal to the central axis. In many applications, radial ejection is most efficient when effected directly along the axis on which two opposing electrodes are positioned. Radial ejection through an electrode requires the electrode to have an ion exit aperture, which is typically shaped as a slot elongated in the axial (z) direction. The slot can be a significant source of field faults that are detrimental to the desired manipulation and processing of ions during certain stages of operation. Therefore, it would be advantageous to eliminate or at least minimize field faults created by slots.
In prior art configurations, the length of the slot is significantly shorter than the overall length of the electrode so that ions being ejected are kept away from the axial ends of the electrode where detrimental field distortions are often pronounced. Various other design considerations have been proposed to minimize the effects of the slot, such as minimizing the size or cross-sectional area (e.g., length and width) of the slot, maximizing the uniformity of the cross-sectional area of the slot, altering other physical features of the electrodes or providing additional physical features to compensate for the presence of the slot, and the like. Despite the foregoing, the mere presence of the slot creates field distortions because the edges of the slot constitute geometric discontinuities. Consequently, the fields active in the vicinity of the slot are different than the fields in other regions of the electrode set. Any differences in a field relative to axial position along the central axis of the electrode set can adversely affect the desired response of the ions and consequently the performance of the electrode set as an ion-processing device. For instance, when the electrode set is employed as an ion-trap mass analyzer, non-uniformity in the field along the central axis can cause ions of the same mass-to-charge ratio to be ejected at different instances of time, resulting in a loss in mass resolution.
In view of the foregoing, it would be advantageous to provide electrode structures for use in ion-processing devices that better address the problems associated with the inclusion of apertures in such electrodes as well as other sources of detrimental field effects in the electrode set, or that improve the uniformity of electric fields generated with the use of the electrode structures.